There are various operations in which drilling is performed, including oil and gas drilling operations and mining operations.
In the process of drilling a well into a subterranean formation, drilling fluid or “mud” is pumped into the developing well bore through the drill pipe and exits through nozzles in the rotating drill bit mounted at the end of the drill pipe. The drilling fluid is then circulated back to the surface through the annulus, the space between the drill pipe and the wall of the well bore. Back at the surface, solids are removed and the mud is generally pumped to a fluid tank where it can be reused or treated. The drilling fluid system is typically designed as a loop with the drilling fluid continually circulating as the drill bit rotates. Drilling fluid performs several important functions and enhances the overall efficiency of the drilling operation. Drilling fluid is used, for instance, to cool and lubricate the rotating drilling tool, to reduce friction, to prevent sticking of the drill pipe, to control subsurface pressure, to suspend and deliver treatment additives, to lift the drill cuttings and carry them to the surface, and to clean the well bore and drilling tool.
A major component of drilling fluid is its base fluid. A drilling fluid may be aqueous based, hydrocarbon based, synthetic based, or an emulsion, such as an oil-in-water or water-in-oil (“invert”) emulsion. Aqueous based, or water based, drilling fluids are used frequently in the industry and provide an economic advantage over oil based drilling fluids. They are also considered more environmentally friendly. Drilling with aqueous based fluids can be problematic in certain formations however, due to hydration and swelling of water-absorbing rock and clay in the formation leading to instability of the well bore. Such hydration and instability can be reduced by using an oil based drilling fluid. Although oil based fluids are more costly than aqueous based fluids, they are generally preferred for deep drilling, high temperature drilling or when a substantially non-reactive base fluid is required for a particular drilling operation or formation.
When drilling into a porous or fractured formation, a portion of the drilling fluid or its base fluid may be lost to the formation surrounding the well bore and is not returned to the surface for recirculation. This lost portion of drilling fluid is commonly referred to as lost circulation. Areas of a formation where losses occur are referred to as loss zones or thief zones. Lost circulation has a negative impact on the drilling procedure and also has a significant economic impact on the operation. Lost circulation can also increase the environmental impact of the operation, particularly when drilling with hydrocarbons. The amount and type of lost circulation experienced depends on the structure and permeability of the formation being drilled as well as the properties of the drilling fluid.
Lost circulation may occur in the form of seepage losses or fluid losses. Seepage losses occur when whole mud, including solids, is lost to the formation during drilling. This can occur when solids in the drilling fluid are not large enough to bridge the pores or fractures in the formation. Fluid loss refers to loss of fluid to an area of a formation where a filter cake is formed or where solids cannot pass, resulting primarily in loss of the base fluid to the formation. Even mild to moderate losses can have a significant impact on the cost of drilling. Severe or total losses may be experienced in highly porous or fractured formations.
Lost circulation can be reduced or controlled to varying degrees by adding foreign solids to the fluid. A solid that is added to a drilling fluid to reduce or control lost circulation is generally referred to as a lost circulation material (LCM). In the field, operators have been known to add a wide range of solids to their fluid systems in an attempt to reduce or control lost circulation, including nut hulls, pieces of rubber tires, dried tumbleweed, paper, and even plastic bags. Conventional lost circulation materials include sawdust, wood fibers, gilsonite, asphalt, asphaltenes, cellulosic materials, plastics, cellophane, calcium carbonate, water soluble polymers and various thickening and gelling agents. Gilsonite, fibrous materials and calcium carbonate are often used to control heavier seepage losses. Various polymers or fine solids are generally added to the drilling fluid to control fluid losses, or to control the thickness of the filter cake created and the amount of filtrate to the formation. Lost circulation materials are often ground or blended to different particle sizes based on the expected structure of the formation and anticipated severity of lost circulation, and are intended to either plug loss zones in the formation with solids or polymers or to build up a mat of solids to seal off the loss zones.
The available lost circulation materials have not proven entirely satisfactory and improved lost circulation materials are desired. Lost circulation materials can have a variety of disadvantages. A significant amount of solid material is often required to control lost circulation and the presence of these solids can cause difficulties in maintaining the desired chemical or physical properties of the drilling fluid, such as yield point, density, surface tension, viscosity or emulsion stability. Oil wetting chemicals must typically be added to ensure the solids are oil wet when drilling with a hydrocarbon drilling fluid, thereby adding chemicals to the fluid. A mat of fine solids can be difficult to keep in place downhole due to the movement of the drill string in the well bore and the annular velocity of the pumping fluid. Many of the available solid additives are not easily removable from the formation and can cause permanent damage. In an oil or gas drilling operation, a significant problem associated with the use of solid lost circulation materials is a permanent damaging effect on production zones of a formation which hinders production of the resource. The use of solid additives can also cause mechanical problems in the drilling rig equipment, such as the fluid pumps and solids control equipment.
U.S. Pat. No. 3,455,390 to Union Oil Company of California discloses a method of treating a well to temporarily seal off a drilled well bore prior to production. Finely-divided (i.e. 1 to 50 micron) oil-soluble wax particles are dispersed in an aqueous treatment fluid, which is then pumped into the drilled well to seal off the formation until the well is brought to production. The fine wax particles are designed such that they will dissolve within 24 to 48 hours in the hydrocarbon present at oil bearing zones of the formation to allow the well to come to production. Surface active agents and emulsifiers are added to aid in dispersion and control the fine particle size. The presence of dissolved wax, surface active agents, emulsifiers and various dissolved polymers in the fluid system impacts the properties of the treatment fluid and renders the fluid expensive to manufacture and maintain. It can also render the fluid system ineligible for full disposal which adds to the cost of completing the operation. Since the additives are oil soluble, they are not suitable for drilling with oil based drilling fluids as they will dissolve in the fluid before completion of the drilling operation. The oil soluble particles also have the potential to contaminate the formation and the production fluid.
U.S. Pat. Nos. 3,302,719, 3,455,390, 3,593,794, 3,601,194, 3,625,892, 3,630,280, 3,684,012, 3,717,204, 3,870,668, 3,882,029, 3,979,304, 3,979,305, 3,989,632 and 4,192,753, also to Union Oil Company of California, disclose variations on their oil-soluble additives for treatment fluids, comprising combinations of wax, resins, polymers, copolymers, surface active agents and emulsifiers for temporarily sealing off an underground formation surrounding a well bore. These additives have the disadvantages set out above.
Other drilling fluid additives comprising wax have been reported wherein reactive materials, such as hydrolytically degradable or swellable materials, are encapsulated in a removable or permeable wax casing to prevent or control the reaction of the core reactive material with its substrate until it has reached a desired location in the formation. The wax casing is designed such that it will melt or dissolve downhole in order to release the functional component of the additive. Examples of reactive materials encapsulated in a removable wax coating are described, for example, in U.S. Patent Publication No. 2006/0276345 to Todd et al., U.S. Patent Publication No. 2006/0122071 to Reddy et al., U.S. Pat. No. 7,063,151 to Nguyen et al., U.S. Pat. No. 4,036,301 to Powers et al., and U.S. Pat. No. 4,770,796 to Jacobs. These references are not directed to additives for controlling lost circulation of drilling fluids.
U.S. Pat. No. 4,704,213 to Delhommer et al., U.S. Pat. No. 4,664,816 to Walker, and U.S. Pat. No. 4,614,599 to Walker, relate to encapsulation of reactive lost circulation materials in a removable casing to protect the encapsulated materials from reacting for a period of time in the well bore. The reactive lost circulation materials are either oil-absorbable or water-absorbable polymers or reactive lime. The removable encapsulating material may be a wax that melts or dissolves downhole to release the active component, thereby allowing the polymers to swell upon contact with the fluid or permitting the lime to react with clays in the formation to form a cement-like product to control losses. The removable wax casing merely serves as a temporary encapsulant for the reactive lost circulation materials inside and does not itself serve as a lost circulation material. The reactive materials used are considered damaging to the formation.
Lost circulation of drilling fluid is one of the most serious and expensive problems facing the drilling industry today. It is, therefore, desirable to provide improved drilling fluid additives for reducing or controlling lost circulation to underground formations during drilling operations. Particularly preferred are drilling fluid additives that are effective yet simple and economical in manufacture and use. Additives that minimize damage to the underground formation or the environment are particularly preferred, as are additives that do not hinder the eligibility of the drilling fluid for full disposal upon completion of the operation. It is also desirable to reduce the number of different additives that must be used in a drilling system order to successfully complete the operation.